The present invention relates to a directional coupler. More specifically, the present invention relates to a directional coupler that can be used effectively in the field of ultra fast communications such as ultra fast switches.
With the recent growth of multimedia, the volume of information being used has increased dramatically. Thus, in ultra fast communication fields such as optical communications, there has been a demand for converters that provide power in the class of gigabits/sec in order to handle such large amounts of information. Using conventional electronic switches for these ultra fast converters, i.e., ultra fast switches, has been difficult. Thus, the development of optical switches capable of ultra fast switching has become an urgent task.
Currently, directional-coupler optical switches are generally used as ultra fast optical switches. Examples of typical directional couplers include uniform xcex94xcex2 directional couplers and reversed xcex94xcex2 directional couplers. In uniform xcex94xcex2 directional couplers, two optical waveguides are arranged on a substrate having electrooptic properties separated from each other by a predetermined distance. A uniform electrode is disposed on each of these optical waveguides. Potentials having opposite signs are applied to the electrodes to modulate the phases of the light waves guided through the optical waveguides, thus adjusting the coupling ratio of these light waves and providing high-speed switching.
In order to make the cross-state xe2x80x9conxe2x80x9d in this type of directional coupler, a coupling length L0 for the optical waveguides must be set to xcfx80/2xcexa (where xe2x80x9cxcexaxe2x80x9d is the coupling coefficient). Production issues made this difficult to achieve.
With reversed xcex94xcex2 directional couplers, two optical waveguides are disposed as described above, and two electrodes are arranged on each of the optical waveguides, giving the appearance that a single uniform electrode has been divided into two parts. Then, potentials are applied to the electrodes so that potentials with opposite signs are applied to the adjacent electrodes on each optical waveguide as well as the electrodes on the two different optical waveguides that face each other. This application of opposite potentials results in modulation of the phases of the guided light, thus adjusting the coupling ratios of the light waves and providing high-speed switching.
With this type of directional coupler, the applied potential can be controlled so that the bar-state is xe2x80x9conxe2x80x9d or the cross-state is xe2x80x9conxe2x80x9d even if the optical waveguide coupling length does not fulfill the conditions described above.
In a further development of the reversed xcex94xcex2 directional coupler, an electrode in four pieces gives the appearance of a uniform electrode, and potentials with different signs are applied to the pieces. The application of opposite potentials to electrodes divided into a plurality of pieces can be easy to control so that the bar-state is xe2x80x9conxe2x80x9d or the cross-state is xe2x80x9con.xe2x80x9d As a result, drive potential (switching potential) can be decreased. Thus, out of the directional couplers described above, it would be desirable to use the reversed xcex94xcex2 directional coupler or the further developed version of the same, based on the advantages described above.
Where a plurality of electrodes are formed in a reversed xcex94xcex2 directional coupler and the like, however, temperature drift can occur due to differences in the sizes and the shapes of the electrodes. This in turn can lead to unstable drive potential. A possible measure to overcome this type of drift is to provide a buffer layer of Si or the like in addition to SiO2 between the substrate and the electrodes. However, providing a thick buffer layer will result in a higher drive voltage. Also, an Si layer will not be able to completely eliminate drift. Furthermore, to provide switching, the signal electrodes must be divided, and production of traveling wave electrode structures is difficult and prevents increases in speed.
An object of the present invention is to provide a new directional coupler that has a low drive potential, provides a complete (100%) xe2x80x9conxe2x80x9d state for the bar-state and the cross-state at ultra fast speeds of several tens of gigahertz or more, and is theoretically capable of eliminating temperature drift.
The present invention provides a directional coupler that includes a substrate having electrooptic properties, two optical waveguides formed in the substrate roughly parallel to a main surface of the substrate, and an electrode arranged on each optical waveguide for modulating light waves passing through the respective waveguides. The substrate is divided into a plurality of reversed polarization domains along a direction of progression of the light waves and the electrodes effectively are divided equally by an extension of a boundary surface between the plurality of reversed polarization domains. As a result, the electrodes are effectively equivalent to two electrodes arranged side-by-side along each optical waveguide.
The directional coupler according to the present invention provides an electrode structure equivalent to that of the reversed xcex94xcex2 directional coupler, but potential can be reduced since division is easier. Additionally, since the polarizations of the adjacent polarization domains are reversed and the electrodes separated by the boundary surface have the same electrode area, there are equal and opposite loads in the electrodes that cancel each other out (i.e., there is no load in the electrodes). Temperature drift is substantially eliminated since there is no load in the electrodes.
As in uniform xcex94xcex2 directional couplers, in the directional coupler according to the present invention, only the uniform electrodes on the optical waveguides actually modulate the light waves. Thus, the electrode structure is simple and traveling wave electrodes can be formed, in contrast to the reversed xcex94xcex2 directional coupler, which uses electrodes on each optical waveguide that are effectively divided in two. Thus, ultra fast switching is made possible with the present invention.
Furthermore, since sets of adjacent domains having the same area and opposite polarization directions are formed in the substrate directly under the electrodes, pyroelectric loads also cancel each other out. This allows temperature drift to be eliminated even if there is a buffer layer. Thus, temperature drift is limited and variations in drive potential can be prevented.
Furthermore, compared to reversed xcex94xcex2 directional couplers with divided electrodes, the directional coupler according to the present invention allows the use of traveling wave electrodes, which are uniform electrodes, and a plurality of effective electrode divisions can be implemented by dividing the substrate into a plurality of reversed polarization regions. Thus, high-speed, low-potential operations are possible and temperature drift can be substantially eliminated.
The present invention also provides a method of making a directional coupler comprising the steps of providing a substrate comprising a material that exhibits electrooptic properties, subjecting the substrate to a polarization treatment to create at least two reversed polarization domains separated from one another by a boundary surface that lies substantially perpendicular to opposed main surfaces of the substrate and substantially perpendicular to the direction of length of the substrate, forming at least two optical waveguides in at least one of the two main surfaces of the substrate, the waveguides extending generally in the direction of length of the substrate and intersecting a planar extension of the boundary surface, and forming an electrode above a portion of each optical waveguide for modulating light waves propagating through the waveguides, respectively, each electrode being divided substantially equally by the planar extension of the boundary surface.